monster sub design advice?

[lbstyling]

hi all,
i just got the deal of the century and picked up a pair of jbl 4641 18" passive (un powered) subs and a pair of cinema surround speakers that are diy from one of the surround mix rooms in pinewood studios here in england.

im keeping one of the subs and planning to build a new box (with a more acceptable shape) -this will be my first diy design thats my own design- i was hoping for any advice you guys can offer on port tuning freq and box sises ?

-i also assume that idealy i will need some electronic assistance to go as low as i can -is there a ideal plate amp or separate amp that has no fan and a parametric eq-would this be enough??

ps tried the sub off a active xo i have through my parasound halo- its effortless and sooo clean-love it!

[lbstyling]

heres the spec

http://www.jblpro.com/pages/cinema/JBL.4641.pdf

just to give you an idea of sise- i can very nealy got my head in the port!!!!!

heres the spec on the drive unit;-

http://www.jblpro.com/pub/components/2241.pdf

[ThomasW]

The JBL box design with it's 25Hz Fb is optimal for this prosound driver.

The driver's short Xmax, typical of old school drivers doesn't lend itself to lower tunings, or using a ton of EQ to force it to play lower in a smaller box.

With the 25Hz Fb, a high pass filter should be used to protect the driver from unloading when DVD's with infrasonic content are being played.

[joecarrow]

My impression that JBL pro gear is well made and thought out. If it were me, the only physical modifications I'd consider would be stiffening the box, probably by gluing a layer of plywood on the outside- but only after first looking inside to see if it's not already reasonably braced, or couldn't be braced.

After that, I'd concentrate on room placement, room treatments, and dialing in EQ for accuracy. Sure these things will play loud, but they'd be more suited to Tchaikovsky than Terminator. They're clean, they have dynamics, an excellent find for you. I hope you enjoy them for what they are- I sure would!

[lbstyling]

thanks joe,and thomas.
i need the box to change shape as its 4ft tall but only 1.5ft deep at the moment. i can accomodate the box being deep -up to a meter or so, but not tall or wider than arround 20" so its got to be a new box.
i had planned to keep the capacity the same, but thought id look into it before i started building, i can eq it flat to 25hz in its current sise/tune and i wish to get it as flat in room as i can anyway so eq is a must.
i dont see the 40mm xmax as an issue too much with a 18" drive unit-i can accept a loss of max volume for a gain of lowend extention imo-i realy dont feel i will need to push 131db+ room gain.
on another note, the sound is very different from my paradigm servo 15- its definatly got that kick drum in the gut feel to it, even at low volumes makes my paradigm sound like a fart! he he.
i take it a sealed enclosure with eq is a no for this driver? id be interested to hear what that would be like.
looking inside theres no bracing and no damping, just a box and a massive port.

[ThomasW]

The driver needs to be in a ported box tuned to ~25Hz.

With it's short Xmax it's not a driver that will work well in a smallish sealed box where EQ is being used to force it to play low.

[minkuni]

...
i dont see the 40mm xmax as an issue too much with a 18" drive unit-i can accept a loss of max volume for a gain of lowend extention imo-i realy dont feel i will need to push 131db+ room gain.

The 40mm xmax rating (+/- 20mm) is a mechanical damage limit. The xmax in ordinary audio terms is only +/- 5.5mm (Height of winding-height of gap)/2. JBL states that the xmax is +/- 7.6mm which I assume includes some leeway in terms of distortion.

on another note, the sound is very different from my paradigm servo 15- its definatly got that kick drum in the gut feel to it, even at low volumes makes my paradigm sound like a fart! he he.

That's what happens when you give a large driver room to breathe :T

On a related note: I was using a pair of JBL 4645C subwoofers as part of a laboratory setup for a school project last autumn. The room they were in was only 2.2x1.8x2 m (a room volume of 280cu ft for the metric challenged :B). The sounds they played were recordings of helicopter rotor noise on a helipad and playback levels were adjusted to 113dBA with peak readings of 130dBC. The system had reasonably flat response down to 20Hz at these levels :twisted:



There is no replacement for short-throw displacement

edit: Reread the spec sheet and added some words on the xmax